Invention relates to data modems, and more particularly to data modem clock synchronization.
Data modems are commonly used to send and receive data via telephone lines or other communications facilities. Each modem has a local clock, which is free running, not synchronized to any other timing source. When two modems first connect, they first must synchronize their local clocks to ensure reliable transfer of data communication. The modems use a variety of clock recovery schemes, such as phase-locked loop circuits, to lock to opposing modem clock. This type of scheme requires a period of time when the modems send certain known sequences of signals, such as alternate ones and zeros and scrambled ones, to allow their respective clock recovery circuits to phase lock to the modem clock at the other end. After this clock synchronization period, the modems begin sending actual data.
In a typical modem connection, clock synchronization time is insignificant when compared to the time it takes for the two modems to transfer data. For typical applications, clock synchronization time is in the range of a few seconds or less, while the actual data communication time is in the order of several minutes or perhaps several hours. However, in applications where data transfer is short and requires only a few seconds or less, then clock synchronization time becomes a significant overhead, particularly when typical synchronization takes as much time, or more, than time to send the actual data. An example of such applications is mobile asset tracking, where an asset is equipped with a GPS receiver to detect its own location, and a wireless modem to report its location to a central monitoring station. Data required to report asset location information, such as latitude and longitude data, is very small (less than 100 bytes), and thus correspondingly involves very little data transmission.
This invention provides data modems having very fast clock synchronization, by having the modem receiver clock synchronize to a precise clock signal derived from a received broadcasted signal comprising a very high precision clock, such as an UTC clock from a global positioning satellite (GPS). The modems connect for the first time using a standard training procedure. Thereafter, once on-line communication between the modems has been disconnected, the modems maintain their local clock synchronization to the GPS""s UTC clock provided to a local GPS receiver. Since GPS receivers at both end of the modem terminals are synchronized to the same received broadcasted signal comprising the same high precision clock from the GPS, this prevents the local modem clocks from drifting apart, thereby significantly reduces the modem synchronization time, or the training time at the next modem connect time.
The GPS receiver provides dual functions of producing a location fix, as well as providing precise timing to a wireless modem. And since the modem at the central monitoring station is also equipped with a GPS receiver, the central modem maintains synchronization with the remote modem using the same precise clock available from its GPS receiver. Because the GPS constellation consists of 24 orbiting satellites that continuously broadcast signals, the signal processor of GPS receiver use these broadcasted signals to determine its own latitude and longitude position. And since each GPS satellite is also equipped with a highly stable Cesium clock, the GPS receivers use this precise clocks to correct their local clock, which is not as stable as the GPS atomic clocks.
Typically, the GPS receivers generate a clock that is locked to the GPS atomic clock and operates at 1 pulse per second (PPS) rate. The accuracy of this clock is bounded by the Selective Availability (SA) which limits the position accuracy of a non-differential GPS receiver to about 100 meters which translates to a clock error of about 333 nanoseconds. This error, when compared to the bit period, and bit clock, of a voice-band modem, is insignificant.